Laser corneal refractive procedures, including photorefractive keratectomy (PRK) and laser-assisted in situ keratomileusis (LASIK), have become popular for the correction of imperfect vision conditions, such as myopia and/or astigmatism. These procedures involve the distribution of carefully controlled laser pulses over the treatment zone to reshape the corneal surface, which alters the refractive power and shifts the focal point of the cornea. Clinical laser systems are technologically advanced and very precise in many ways, allowing removal of corneal tissue with submicron accuracy. However, presently, the precision of refractive measurements exceeds the precision of tissue removal via ablation, hence real-time monitoring and control of the ablation rate remains a needed technology.
However, the hydration state of the eye, the healing response of the particular patient, the mechanics of the retina and other factors affect the results obtained. Furthermore, the refraction is conventionally achieved through a pre-computed process that attempts to account for all of the various factors. However, the pre-computed processing is entirely, or at least primarily, based on large patient population averages. The goal is to provide a process that achieves the desired change of refraction while minimizing the optical aberrations of the ocular system. However, in many cases the refractive surgery itself introduces significant aberrations. This may be due to either a change in the bio-mechanical structure, due to the edge of the treatment zone, due to variations in the actual patient-to-patient ablation rates, or due to variations across an individual cornea.
A number of instruments have been developed that have served as diagnostics for this process. These includes subjective and auto-refraction, corneal topography, pachymetry, and wavefront aberrometry. However, there are still variables that cannot be properly monitored that affect the outcome. In particular, factors such as the hydration state of the eye and healing response are difficult to account for in advance. The ablation profile for LASIK has long been known to be non-linear and have a different strength for positive or negative corrections. The ablation algorithms have been developed to take this into account, but there is still considerable variation from subject to subject because of largely unknown factors.
If the diagnostics could be applied in real time during the refractive surgery, some of this variation could be removed. This would allow the laser surgery to operate in a closed-loop mode, with the amount of refractive modification being monitored and controlled during the procedure. While it may be possible to monitor the change in shape of the cornea in “real-time” with corneal topography or other surface measurement apparatus, this only indirectly affects the total optical path and hence the refraction and higher order terms.
Frey, Burkhalter, Zepkin, Poppeliers and Campin in U.S. Pat. Nos. 6,271,914 and 6,271,915 disclose a method for ablating corneal material while monitoring the process in real time using a Hartmann plate sensor. Unfortunately, their techniques rely on modifying directly the optical zone that is measured. During the LASIK or PRK procedures that use ablation of portions of the cornea, the process of ablating material leads to unknown and undetermined optical scattering and effects during the ablation process. The surface of a dry cornea (needed for properly controlled ablation) or the interior surface that is exposed during the LASIK procedure are inherently rough. Thus these surfaces would scatter the injected and reflected light that is used for monitoring the wavefront. This significantly degrades the quality of the information obtained, making the aim difficult to achieve.
Published U.S. Application No. 20030174281 to Herekar et al. discloses a refractive control system including a laser refractive surgery instrument for modifying the refraction of the eye, an objective diagnostic apparatus for measuring the refraction and aberrations of the eye, and an aperture-sharing element to inject a refractive surgery beam and a monitoring diagnostic beam. Information regarding the refraction of the eye during surgical procedures is used to force the laser system to shut off when the desired refraction is achieved. Although Herekar et al. seek to take advantage of optical aberrations to provide feedback, they acknowledge that “in many cases the refractive surgery itself introduces significant aberrations” and that there is “a significant difficulty with incorporating the diagnostics into the lasers that are used for LASIK or PRK” because of obvious optical aberrations that are realized in these procedures. They further state that “while it may be possible to calibrate for these effects, it certainly falls short of the goal of directly measuring the desired result in real-time during the procedure.” Accordingly, Herekar et al. relates to endpoint determination, and regarding the admittedly deficient real time feedback embodiment, does not disclose or suggest real-time feedback based on ablation rate, or a parameter correlated with the ablation rate.